c# - Use extern c++ dll in Unity3D 5 -

i wrote c++ dll want use in unity3d. put assets/plugins , wrote c# script should access function of error:

entrypointnotfoundexception: multiply
test.start () (at assets/scripts/test.cs:11)
manager.h:

extern "c" {     class manager     {     public:         __declspec(dllexport) float multiply(float a, float b);     }; } 

manager.cpp:

 #include "manager.h"   extern "c" {     float manager::multiply(float a, float b)     {         return * b;     } } 

c# script:

 [dllimport("manager", callingconvention = callingconvention.cdecl, charset = charset.unicode)]     private static extern float multiply(float a, float b);      void start()     {         debug.log(multiply(10, 2));     } 

edit: doesn't work during i'm using class. use class have this